Abstract: According to the optical compensation theory, a 31 infrared step zoom optical system was designed based on 320240 cooled midwave infrared (MWIR) focal plane array (FPA) detector. Its operation wavelength range was 3 m~5 m, F number was 4, cold stop efficiency was 100%, and focal length range was 430 mm~12.867 mm. The step zoom optical system was optimized by adopting three kinds of optical materials and aspheric surface. Aberrations of seven fields of view (FOVs) were strictly corrected, the MTF of each FOV was close to diffraction limit, and the image quality was good.